1. Field of the Invention
The present invention relates to a code converting device for converting first code signals into second code signals, and more particularly to a code converting device for converting code signals of a certain bit number into other code signals of a smaller bit number.
2. Description of the Prior Art
Recent computer output with Japanese characters requires 14-bit or 16-bit character codes such as JIS C 6226 standard code instead of conventional 7-bit or 8-bit character codes such as JIS C 6220 standard code. A 16-bit character code system can represent 2.sup.16 =65,536 characters, all of which are however seldom used since actually used characters are limited to 3,000 to 8,000 species. Although a 13-bit character code system corresponding to 2.sup.13 =8,192 codes is theoretically sufficient for representing 8,000 characters, the 16-bit code system is in common use because of the commercial practice or in consideration of the future possibility. For this reason 8,000 characters are suitably allotted in 65,536 character codes.
FIG. 1 shows an example of 16-bit character code system in practical use, in a 8.times.8 bit map representing the upper or more significant 8 bits in the column direction and the lower or less significant 8 bits in the digit direction. In FIG. 1, the area 100 indicates the entire code area of 65,536 species which can be represented by a 16-bit code system, while the areas 101, 102, 103 and 104 indicate the character code areas actually used. More specifically the character code system shown in FIG. 1 consists of 7,148 codes, i.e. 658 codes of 2121-217E, . . . , 2721-277E in the area 101, 6,392 codes of 3021-307E, . . . , 7321-737E in the area 102, 1 code of A0A0 in the area 103 and 96 codes of B040-B09F in the area 104, all the addresses being in hexadecimal representation. In this manner the character codes are quite dispersely scattered or uncontinuous. In such system, any other code does not represent any actual pattern.
The use of such dispersed character codes as the addresses of a character pattern memory character generator directly or after arithmetic processing gives rise to a wasted space in the pattern memory, requiring an additional capacity therein. Consequently there has been employed a code converting device for converting such dispersed character codes into consecutive character codes for use in access to the character pattern memory. Such converter theoretically requires a memory of a capacity capable of storing a converting table of 2.sup.16 =65,536 words in order to convert a 16-bit code system into another. In practice, however, the use of such a large memory is quite uneconomical since the number of character codes in actual use is limited to about 8,000 as explained before.
Although the foregoing explanation has been limited to the case of Japanese characters, a similar situation is encountered also in case of coding graphics or patterns.